I think I tried compiling with 1.4.2 and 1.5 and comparing the results a few contest ago. If I remember correctly, there was not much of a difference.

That being said, I usually target 1.4.2 but using some 1.5 syntax that gets lost in compilation (generics, for one). On occasion I target 1.5, but usually not. I don't think targetting 1.6 is such a good idea, since many users apparently do not run JRE6.

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